In the preparation of a nickel electrode by impregnating nickel hydroxide from a bath into a porous plaque, the pH (or relative acidity) of the bath is a critical operating parameter. (As used herein, the term nickel hydroxide is intended to include nickelous hydroxide and nickel oxy-hydroxide.) Bath pH should be acid, in the approximate range of 1.8 to 3.5. Above this range, the bath is insufficiently acid, the hydroxide tends to deposit on or near the surface of the plaque, and hydroxide may start to precipitate in the bath itself. Below this range, the plaque can be corroded by the extreme acidity, and the acid attacks the nickel hydroxide itself. It is therefore necessary to provide some means to maintain pH in this range as impregnation proceeds. This invention is directed to a new, more efficient, and less expensive means of controlling bath pH within the desired range.
In electrolytic impregnation a plaque, which is usually of porous metal such as sintered nickel, is immersed in a bath containing a source of a nickel salt such as nickel nitrate (Ni(No.sub.3).sub.2) or the like, and is connected as a cathode in a circuit which includes a source of direct current and an anode. The anode may be of consumable nickel, for example, metallic nickel sheet or rod, scrap nickel, or a consumable nickel oxide. Often the nickel source is contained in an electrically conductive basket of an inert (nonconsumed) material. As direct current is passed from the anode through the bath to the cathode, the nickel salt in the bath is converted through a series of reactions into nickel hydroxide which is deposited within the porous plaque. The general technique of electrolytic deposition is further described in Kandler U.S. Pat. No. 3,214,355, issued Oct. 26, 1965, to which reference may be made for more complete background discussion.
As the deposition of nickel hydroxide proceeds, the pH of the bath, particularly in the region around the cathode, tends to increase. If steps are not taken to maintain pH below about 3.5, and preferably below about 3.0, it eventually becomes undesirably high, with consequences as noted above. Ultimately the deposition will essentially stop, and an inferior electrode will result.